The proposed project is an attempt to achieve a better understanding of the neural basis of biological rhythms, specifically those with periods of approximately 24 hours (circadian rhythms). In order to identify the neural structures and pathways involved in the generation of circadian periodicity and the entrainment of circadian rhythms to the light-dark cycle, lesions and knife cuts will be placed in selected hypothalamic and midbrain structures of rats. The effects of these manipulations on rhythmicity in food and water intake, sleep and activity, and body temperature will be assessed. In addition, the animals will be exposed to constant light, darkness, and phase shifts in the light-dark cycle. These investigations will be complemented by neuroanatomical methods.